Current models of countercurrent multiplication by the thin limbs of Henle all agree that osmotic equilibrium between the luminal contents and the interstitial fluid exists in the descending limb of Henle. There is also wide agreement that preferential solute reabsorption occurs in the thin ascending limb of Henle. The precise mechanisms of these two processes are somewhat controversial. Considerable disagreement exists over what fraction of osmotic equilibiration in the descending limb is due to water abstraction. Experiments are planned which will explore not only the osmotic effect of NaCl on in vitro descending limbs, but also the identification of which membrane (luminal or basolateral) best explains the apparent water permeability properties of this segment and whether or not tubules not permeable to water are sensitive to the antidiuretic hormone. Whether or not "preferential solute absorption" by the thin ascending limb can be completely explained by passive mechanisms also remains controversial. The planned studies of the transport and bioelectric properties of both thin descending and thin ascending limbs will permit an evaluation of the existing "passive" models for preferential solute reabsorption in the thin ascending limb of Henle. A second area of proposed research is the use of collapsed renal tubule fragments to study the properties of the basolateral membrane. By analyzing the Na+ and K+ contents of renal tubule fragments the net movement of cations can be studied under a variety of experimental conditions. Protocols include: 1) a study of the effects of basolateral PH on the K+ content of rabbit collecting tubule, 2) the identification of a Na+ mediated glucose uptake mechanism by the basolateral membrane of the pars recta, 3) the effect of aldosterone on the ability of the cells of cortical collecting tubules to actively extrude Na+, and 4) the relationship between the concentration of organic molecules in the basolateral medium and cationic volume regulatory mechanisms of the pars recta. It is anticipated that this methodology will for the first time permit the investigator to study a variety of cation transport mechanisms found in the basolateral membrane of precisely defined fragments of the renal tubule.